1. Field of the Invention
This invention pertains to the field of weather radar and more specifically to an apparatus for the suppression of unwanted terrain generated radar backscatter signals with minimal alteration to weather backscatter signals.
2. Description of the Prior Art
Airborne weather radar systems are specifically designed to detect and display rainfall intensities when flying at various altitudes. In such systems the antenna beam may be aimed downward since the rain appears below the freezing level; consequently the rain backscatter signals may become cluttered by ground terrain signals which can appear in the antenna mainlobe or sidelobes at the same range as the storm cell. The backscattered terrain signals may alter the intensity of the backscattered signals received from the storm cell or even obscure them altogether. It is therefore desirable to be able to separate the storm cell backscattered signal from the terrain backscattered signal so that the weather signal may be analyzed with a minimum of alteration.
Prior art attempts to isolate the weather signal from terrain clutter have been largely unsuccessful. A major portion of the scintillation frequencies from terrain radar returns fall below 50 Hz while those from storm cell radar returns are in the range of 50 to 450 Hz. One method for isolating frequencies in the range of 50 to 450 Hz from those in the region below 50 Hz may comprise a filtering technique. Simple lumped constant analog filtering techniques, however, are not applicable to the return from pulsed radar, due to the brief duration of the samples available as a video signal in such a system. Lumped constant filtering techniques are generally too costly to implement to achieve good results. Attempts using high frequency doppler radar to differentiate between a moving storm cell and stationary terrain have been unsuccessful since the storm cells do not move quickly enough. Another method which has been used includes reading the clutter signal in isolation, storing the clutter signal, and subtracting the stored clutter from the real time combined weather plus clutter signal. This method may be inaccurate or difficult to implement since it is difficult to take a reading to the terrain without the presence of the backscattered weather signal and it cannot accurately take into account changes in the terrain backscattered signal. The present invention provides a means by which the separation of weather (rain) backscattered and terrain backscattered signals may be realized to permit the detection of rainfall intensities with a minimum of interference from terrain reflected signals.